This invention relates to wireless subscriber systems and, more particularly, the use of devices for granting or denying access to wireless subscriber systems to prevent cellular telephone cloning fraud.
Fraudulent or other unauthorized users reprogram clone phones with the MIN (mobile identification number) and ESN (electronic serial number) from authorized phones of subscribers. This cloning permits the unauthorized users to place cellular phone calls without paying for them. A technique is needed to distinguish between cloned phones and authorized phones so that steps can then be taken to prevent the cloned phones from having access to the cellular phone system.
In the Magril (U.S. Pat. No. 5,329,591), Ferrell patent (U.S. Pat. No. 5,005,210), and Fredrick patent (U.S. Pat. No. 5,448,760) patents, systems are provided for preventing access by unauthorized users. Neither the Magrill, Ferrell, nor Fredrick patents takes into account the RF propagation effects that distort the transmitted waveform from the wireless subscriber unit. Transmitters that are a long distance away from the identification system will have a low signal-to-noise ratio. In cellular telephone applications, frequencies are reused which increase the probability of co-channel interference from a nearby cell. Some wireless subscriber units attempt to access the wireless subscriber system at the same time as other units thereby causing co-channel interference.
A fraudulent user may attempt to gain access to a wireless network several consecutive times within a short period of time. Such repeated attempts may be detected or may result in the fraudulent user gaining access to the system. Prior systems do not use the results of previous decisions on access to aid in the current decision as to whether or not the current attempt is legitimate.
Prior systems do not provide a technique of fraud detection using the outputs of diversity and sectorized antennas typical of cellular base stations.
Although the Ferrell system extracted a few transient parameters to characterize transmitters and may have worked in the environment that consisted of a few ham radio transmitters, a need exists for the ability to distinguish between thousands of legitimate cellular phones and unauthorized phones.
Accordingly, there is a need for improved fraud prevention systems having parameters that are effective in distinguishing between authorized cellular phones and unauthorized clones.